Various micromirror devices have been devised as micro-mechanical structures to which micromachining technology is applied, and their applications to optical scanning apparatuses such as laser printers, scanners, and displays, and optical switching devices to/from which input/output of optical fibers is switched are under study.
Patent Document 1 discloses a reflection-type optical modulation device having an array of micromirrors. This device has been developed as an image displaying device of a video projector, and is known as a DMD (Digital Micromirror Device). Micromirrors of the DMD are formed on CMOS circuitry.
The DMD which is disclosed in Patent Document 1 includes: a bias bus and a pair (left-right) of address electrodes as metal members in a first layer (lowermost layer) upon an oxide film; a twist hinge, a pair of hinge support portions, a yoke, and a pair of elevated electrodes as metal members in a second layer (intermediate layer); and a mirror as a metal member in a third layer (uppermost layer). A positive type photoresist, which is stacked as spacers in between layers, is finally removed by plasma etching. As a result, the yoke and the mirror are pivotably supported by the twist hinge, and are driven so as to pivot in two directions due to an electrostatic attraction that is caused by a difference in potentials applied to the address electrodes and the yoke. A multitude of mirrors are disposed in a two-dimensional array, such that each mirror functions as a pixel and allows light from a light source to be projected toward a screen, whereby a video image is displayed.
The structural parts such as the mirror, the electrodes, the twist hinge, and the yoke are formed from thin films which are obtained by depositing aluminum or an aluminum alloy by sputtering technique. Since each such thin film has a stress gradient along the film thickness direction, there is a problem in that the mirror may be warped when the spacer layers are removed during the production steps.
Since the twist hinge is to be driven by a low voltage, it is thinly formed so as to lower its torsional rigidity. However, the yoke needs to have sufficient rigidity to ensure that the yoke will pivot integrally with the mirror when tilting under an electrostatic attraction, without deforming itself. Therefore, although the twist hinge and the yoke are formed so as to be at the same height from the substrate, they are formed with respectively different thicknesses. Therefore, a step is performed which involves forming a thin aluminum layer composing the twist hinge, and thereafter depositing a first oxide film and patterning it into a hinge shape. Then, using the first oxide film as an etch stop, a removal through plasma etching is performed to also remove a thick yoke layer that is formed thereon. Thus, the shapes of the twist hinge and the yoke are defined. The mirror which is formed on the yoke is linked via a mirror supporting post which is provided in the central portion. Since the mirror and the mirror supporting post are formed of the same aluminum alloy film, a dent of a shape corresponding to the mirror supporting post is formed in the mirror surface.
Non-Patent Document 1 discloses an optical-spatial modulator in which an array of low-inertia micromirrors that are bonded onto actuators are disposed. This construction adopts a method where a plurality of comb-type actuators are used in combination to tilt a central base, with separately-produced mirrors being bonded thereon. An optical modulation is performed by moving up/down or tilting each mirror in the micromirror array of such a construction, whereby wave aberration correction for reflected light, etc., can be performed. For such purposes, the aberration of the individual mirrors themselves may become a problem, and therefore high-precision mirrors having a high planarity are required. Hence, the mirrors and the actuators are fabricated by using an SOI (Silicon-On-Insulator) substrate. By nature, a single-crystalline silicon is free of internal stress, and can produce mirrors with a good planarity.
Furthermore, through a 3-level selective DRIE process, the mirrors are structured so as to have vertical ribs on their rear faces, thus obtaining light-weighted and highly rigid mirrors. Thus, not only rapid response is enabled, but also each mirror is prevented from being warped under the influence of a difference between the coefficients of thermal expansion of an aluminum reflective layer that is provided on the mirror surface and the main body of the mirror.
As for bonding between each actuator chip and each mirror chip, methods are possible such as: a eutectic bonding which involves forming gold bumps or gold-tin solder on the bonding surface; a polymer bonding which involves forming a polyimide film and heating it to about 350° C. while applying a voltage thereto; and so on.
[Patent Document 1] Japanese Laid-Open Patent Publication No. 8-334709
[Non-Patent Document 1] Tip-tilt-piston Actuators for High Fill Factor Micromirror Arrays (Solid-State Sensor, Actuator and Microsystems Workshop, 2004 Adriatic Research Institute)